1. Field of the Invention
The present invention relates to a noise suppression system for jet engines whereby a plurality of microjets pulse a microjet output at the main exhaust stream emanating from the microjet, the microjets being located either directly on the aircraft or externally of the aircraft.
2. Background of the Prior Art
Although jet engines have become quieter over the years, noise produced by jet engines continues to be a problem for many especially at and near airports where the jet plane is on or near the ground and where jet pressure is high either to achieve V-2 takeoff speed or for reverse thrust to brake an airplane. As many airports are located in urban and suburban areas with development constantly encroaching, jet engine noise is a sizable problem. Efforts are continually being made to reduce jet engine noise at and around airports. Similarly, on an aircraft carrier, a jet aircraft typically goes to maximum engine thrust just prior to aircraft launch in order to have sufficient air speed to sustain flight immediately after launch. This maximum thrust results in a very high noise output.
The far-field noise of a supersonic jet is comprised of four major noise components. The first noise component is a high frequency short wavelength field that is coherent in phase and is commonly referred to as Mach waves. These Mach waves have plane phase fronts and are confined to a definite wedge sector and emanate from a region within the first few exhaust diameters downstream of the jet exhaust nozzle exit. These waves are generated by small-scale disturbances, or eddies, that are convected at supersonic speeds such that they emanate the Mach waves in the direction defined by a disturbance convection velocity and the atmospheric speed of sound. Surrounding these waves by a gas stream that has a speed that is greater than the speed of sound eliminates these waves.
The second noise component is a highly directional disturbance peaking at smaller angles relative to the jet axis (or at larger angles relative to the inlet axis). This noise field is generated from large-scale instabilities that reach peak amplitude in the region that is somewhat upstream of the end of the potential core. This source of noise is associated with the unsteady flow that is on a scale that is comparable with the local shear layer width. The spectral intensity of this sound field consists of two distinct peaks. One peak is associated with the highly directional Mach waves characterized by high positive pressure peaks in the far-field microphone signal. These Mach waves are of significant strength as compared to those that originate very close to the jet exit. This intense radiation is observed to emanate from a region that is between about 5-10 jet nozzle diameters and is associated with supersonically traveling large-scale coherent regions of vorticity. The far-field intensity contributions of this source is about 30 percent of the total intensity. The sources of the second sound field peak appear to located farther downstream (about 10-20 nozzle diameters) and are associated with unsteady flow generated by the large structures that are similar to those in subsonic jets.
The third noise field is at all angles relative to the jet axis and is at higher frequencies. This sound is generated in precisely the same manner as in subsonic flow by the conventional chaotic turbulence.
The fourth noise field is commonly referred to as shock-associated noise and it occurs in non-ideally expanded jets. The far-field noise spectrum associated with this noise typically consists of discrete peaks which represent the screech tones and a broad peak that is associated with the shock-associated broadband noise.
Attacking these noise components helps reduce the noise output from a jet engine.